U.S. patent number 4,793,000 [Application Number 06/920,130] was granted by the patent office on 1988-12-20 for light signal receiver.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Izumi Ichikawa, Haruo Imano, Genmei Miura, Satoshi Ogiwara.
United States Patent |
4,793,000 |
Imano , et al. |
December 20, 1988 |
Light signal receiver
Abstract
A light signal receiver comprises a photo-sensing element for
sensing a modulated light signal, conversion means for converting
the light signal sensed by the photosensing element to an
electrical signal, and resonance means having a filter element or
resonator connected in parallel with a load resistor of the
photo-sensing element.
Inventors: |
Imano; Haruo (Yokohama,
JP), Ichikawa; Izumi (Atsugi, JP), Ogiwara;
Satoshi (Atsugi, JP), Miura; Genmei (Tokyo,
JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
26531629 |
Appl.
No.: |
06/920,130 |
Filed: |
October 17, 1986 |
Foreign Application Priority Data
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|
|
|
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Oct 22, 1985 [JP] |
|
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60-234555 |
Oct 22, 1985 [JP] |
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60-234556 |
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Current U.S.
Class: |
398/202;
250/214R |
Current CPC
Class: |
H04B
10/69 (20130101) |
Current International
Class: |
H04B
10/158 (20060101); H04B 10/152 (20060101); H04B
009/00 () |
Field of
Search: |
;250/214A,214R,206
;307/311 ;331/66 ;328/2 ;455/619,617 ;329/117-118,144 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Westin; Edward P.
Assistant Examiner: Shami; Khaled
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Claims
We claim:
1. A light signal receiver for detecting information carried on a
modulated light signal, said receiver comprising:
light-sensitive receiving means for receiving the modulated light
signal;
converting means for converting the light signal received by said
receiving means to an electrical signal; and
resonance means connected to said receiving means and said
converting means, for providing a resonant frequency in said
receiver, said resonance means having a mechanical vibration
element whose resonant frequency is tuned to a modulation frequency
of the received signal, thereby to improve detection of the
information carried on the modulated light signal.
2. A light signal receiver according to claim 1, wherein said
receiving means is a photo-sensing element, and said resonance
means is a resonance circuit having the mechanical vibration
element connected in parallel with a load resistor of said
photo-sensing element.
3. A light signal receiver according to claim 1, wherein said
receiving means is a photo-sensing element, and said resonance
means is a resonance circuit having a crystal resonator connected
in parallel with a load resistor of said photo-sensing element.
4. A light signal receiver according to claim 1, wherein said
receiving means is a photo-diode.
5. A light signal receiver comprising:
a photo-sensing element for sensing a modulated light signal;
conversion means for converting the light signal sensed by said
photo-sensing element to an electrical signal; and
resonance means having a filter element connected in parallel with
a load resistor of said photo-sensing element.
6. A light signal receiver comprising:
a photo-sensing element for sensing a modulated light signal;
conversion means for converting the light signal sensed by said
photo-sensing element to an electrical signal; and
resonance means having a resonator connected in parallel with a
load resistor of said photo-sensing element.
7. A light signal receiver according to claim 1, wherein said
receiving means is a photo-sensing element, and said resonance
means is a resonance circuit having a ceramic vibrating element
connected in parallel with a load resistor of said photo-sensing
element.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light signal receiver for
converting a modulated light signal to an electrical signal.
2. Related Background Art
A light signal receiver has been widely used as an apparatus for
detecting a change in a moving article or a material which exists
on an optical path, and recently, it has been used as a light
communication apparatus for transmitting various information by
optical means by using a space or optical fiber as a transmission
medium. A photo-sensitive element such as a photodiode or avalanche
photo-diode has been used as a photoelectric conversion element for
converting the light signal to the electrical signal.
FIG. 3 shows a specific circuit of a prior art light signal
receiver.
In FIG. 3, numeral 1 denotes a photo-diode. A backward voltage +B
is applied to a cathode of the photo-diode 1, and a load resistor 3
of the photo-diode 1 is connected to an anode. The junction between
the photodiode 1 and the load resistor 3 is connected to an input
terminal of an amplifier 2 through a coupling capacitor 5. Numeral
6 denotes an output terminal of the amplifier 2 and numeral 4
denotes a bypass capacitor for the backward voltage +B. In this
light signal receiver, a light signal applied to the photo-diode 1
is photo-electrically converted to a signal current which flows
through the load resistor 3 to develop a voltage across the load
resistor 3. The developed voltage is applied to the amplifier 2
through the coupling capacitor 5, where it is amplified and a
signal output is produced at the output terminal 6. The input
impedance of the amplifier 2 is sufficiently larger than the load
resistor 3.
When the light signal is to be detected in such a circuit, it is
necessary to increase the load resistor 3 in order to improve
receiving sensitivity. However, it has been experimentarily proved
that the sensitivity saturates as the load resistor 3 increases, as
shown in FIG. 4.
Further, because of a junction capacitance C.sub.j (shown by broken
lines in FIG. 3) of the photo-diode 1, the signal output level
decreases as the signal frequency increases, and this leads to
reduction of the sensitivity of the light signal receiver.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved
light signal receiver.
It is another object of the present invention to eliminate defects
of the prior art apparatus.
It is still another object of the present invention to improve
receiving sensitivity for a light signal.
It is still another object of the present invention to provide a
light signal receiver whose receiving sensitivity does not decrease
with the signal frequency of a light signal.
It is still another object of the present invention to match a
light modulation signal frequency to a resonance frequency of a
resonance circuit to produce a high power signal output having an
excellent frequency selection characteristic.
It is still another object of the present invention to provide a
resonance circuit having a filter element connected in parallel
with a load resistor of a photodiode which serves as a
photo-sensing element for a light signal.
It is still another object of the present invention to provide
resonance circuit having a resonator (oscillator) connected in
parallel with a load resistor of a photo-diode which serves as a
photo-sensing element for a light signal.
Other objects of the present invention will be apparent from
consideration of the following description of the preferred
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a circuit diagram of a first embodiment,
FIG. 2 shows a sensitivity enhancement characteristic in the first
embodiment,
FIG. 3 shows a prior art circuit,
FIG. 4 shows an output level characteristic for each frequency in
the prior art circuit,
FIG. 5 shows a circuit diagram of a second embodiment,
FIG. 6 shows a circuit diagram of a third embodiment,
FIG. 7 shows an output level characteristic for each frequency in
the prior art circuit, and
FIG. 8 shows a circuit diagram of a fourth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will now be
explained with reference to the drawings.
In a first embodiment of the present invention, a resonance circuit
is constructed by parallelly connecting a filter element with a
load resistor of a photo-diode which serves as a photo-sensing
element.
FIG. 1 shows a circuit diagram of the first embodiment. Numeral 1
denotes a photo-diode. A backward voltage +B is applied to a
cathode of the photo-diode 1, and a load resistor 3 of the
photo-diode 1 and a crystal resonator 7 are connected to an anode.
The crystal resonator 7 serves as a filter element in this
embodiment. The junction of the photo-diode 1 the, load resistor 3
and the crystal resonator 7 is connected to an input terminal of an
amplifier 2 through a coupling capacitor 5. Numeral 6 denotes an
output terminal of the amplifier 2 and numeral 4 denotes a bypass
capacitor for the backward voltage +B. The input impedance of the
amplifier 2 is sufficiently higher than the load resistor 3.
The present circuit is characterized by the parallel connection of
the ceramic filter element 7 with the load resistor 3. In the
present circuit, an equivalent circuit viewed from the input
terminal of the amplifier 2 toward the photo-diode 1 forms a
resonance circuit. When the modulation frequency of a light signal
matches the resonant frequency of the resonance circuit, the
resonance impedance of the resonance circuit is very high.
Accordingly, an input level to the amplifier 2 is higher than an
input level obtained when the ceramic filter element 7 is not
connected. As a result, the level at the output terminal 6 of the
amplifier is also high, and the sensitivity of the light signal
receiver is improved.
FIG. 2 shows experimental values of the receiving sensitivity of
the first embodiment shown in FIG. 1.
In the first embodiment, the receiving sensitivity is improved at
any frequency by properly selecting the resonance frequency.
In the first embodiment, the photo-diode is used as the
photo-sensing element. A similar improvement may be attained by
using an avalanche photo-diode. The ceramic filter element is used
as the filter element. A similar improvement may be attained by
other filter element such as crystal filter or SAW filter.
FIG. 5 shows a second embodiment which is a modification of the
first embodiment. As shown, a similar advantage to that of the
first embodiment is attained in the second embodiment, and the
first embodiment can be applied to all circuits which use
photo-diodes or avalanche diodes as the photo-sensitive
elements.
As the sensitivity is improved, the signal to noise ratio is
improved accordingly, and the filter element which has a frequency
selection characteristic can bring an excellent frequency
characteristic to the high sensitivity light signal receiver. Since
the sensitivity is improved, weak light can be detected, and a
simple and inexpensive light signal receiver having a high
sensitivity characteristic can b constructed by merely connecting
the filter element to a conventional light signal receiver.
As explained above, the present embodiment can be applied to all
circuits which use photo-diodes or avalanche photo-diodes as the
photo-sensing elements in the light signal receiver.
A third embodiment in which a resonator is connected in parallel
with a load resistor of a photo-diode which serves as the
photo-sensing element is now explained. FIG. 6 shows a circuit
diagram of the third embodiment. Numeral 1 denotes a photo-diode. A
backward voltage +B is applied to a cathode of the photo-diode 1,
and a load resistor 3 of the photo-diode 1 and a crystal resonator
9 are connected to the anode. A junction of the photodiode 1, load
resistor 3 and crystal resonator 9 is connected to an input
terminal of an amplifier 2 through a coupling capacitor 5. Numeral
6 denotes an output terminal of the amplifier 2 and numeral 4
denotes a bypass capacitor for the backward voltage +B. The input
impedance of the amplifier 2 is sufficiently higher than the load
resistor 3.
The characteristic of the above circuit is that the crystal
resonator 9 is connected in parallel with the load resistor 3. In
this circuit, an equivalent circuit as viewed from the input
terminal of the amplifier 2 toward the photo-diode 1 forms a
resonance circuit. When the modulation frequency of the light
signal matches the resonant frequency of the resonance circuit, the
resonance impedance of the resonance circuit is very high and an
input level to the amplifier 2 is higher than that obtained when
the crystal resonator 9 is not connected. Accordingly, the output
level at the output terminal 6 of the amplifier increases. As a
result, the sensitivity of the light signal receiver is improved.
In FIG. 6, like numerals to those shown in FIG. 1 designate like
elements.
FIG. 7 shows experimental data of the improved receiving
sensitivity in the third embodiment shown in FIG. 6.
In the third embodiment, the receiving sensitivity is improved at
any frequency by properly selecting the resonance frequency.
In the third embodiment, the photo-diode is used as the
photo-sensing element. A similar improvement may be attained by
using an avalanche photo-diode.
While the crystal resonator is used as the resonator, a similar
advantage may be attained by using any other resonator (such as a
ceramic resonator).
FIG. 8 shows a fourth embodiment which is a modification of the
third embodiment. As shown, a similar advantage to that of the
third embodiment is attained in the fourth embodiment.
As the sensitivity is improved, the signal to noise ratio is
improved accordingly. Since the resonator has a high Q, the light
signal receiver can have an excellent frequency selection
characteristic. When the crystal resonator is used as the
resonator, a very sharp frequency selection characteristic is
attained. Since the sensitivity is improved, weak light can be
detected. A simple and inexpensive light signal receiver having a
high sensitivity characteristic can be constructed by merely
connecting the resonator to the conventional light signal
receiver.
The present embodiment can be applied to all circuits which use the
photo-diodes or avalanche photodiodes as the photo-sensing elements
of the light signal receiver.
In accordance with the present invention, a light signal receiver
having a high sensitivity and an excellent frequency selection
characteristic is provided.
* * * * *